1. Field of the Invention
The invention relates to a biomedical optical device and a biomedical optical measuring method for use in measurement of biomedical internal information using light noninvasively.
2. Description of the Related Art
Various diagnostic techniques are known for examining the inside of a body. One of them is optical measurement, which is advantageous in that a compound to be measured can be selected by tuning a wavelength without any problem of radiation exposure. The device can be also reduced in size and lowered in cost, and it is being developed into products for home use or non-clinical use, while other diagnostic devices are mainly designed for clinical and professional use.
Biomedical information measuring devices making use of optical measurement (hereinafter, referred to as biomedical optical devices) are already manufactured in certain commercial products such as sphygmograph (pulse oximeter) or Optical Topography (registered trademark). The latter is to monitor the consumption trend of oxygen, and its subject is mapping of time course of hemoglobin and myoglobin in brain and muscle. Formerly, a handy oxygen monitor was distributed, and it is no longer manufactured at the present.
As compared with the existing established diagnostic devices, such as an X-ray diagnostic apparatus, an X-ray CT apparatus, a magnetic resonance imaging (MRI) apparatus, an ultrasound echo, and a nuclear medicine (positron) diagnostic device, the market scale of optical biomedical measuring devices is smaller. The technology of biomedical optics itself has been known for 30 years, and has attracted wide attention several times in the past. In spite of the biomedical “boom” both inside and outside Japan at the present, corresponding market for products is not formed yet.
A configuration of a conventional biomedical optical device is explained. For example, in a general biomedical optical device, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-237195, an optical probe is pressed to the skin surface of a body, detection light is irradiated into the body through the skin, transmitted or reflected light is transmitted again through the skin, the exit light is measured, and various items of biomedical information are calculated. From the measured light, the position and depth of measurement are analyzed. This analytical technique includes a technique of adjusting a distance (abbreviated to be R) between a light source and a detector (spatial resolution method), and a technique of obtaining depth information from difference in light arrival time using a light source changing in intensity in the time course (time resolution method).
The former spatial resolution method is disclosed in, for example, Appl. Opt. Vol. 34, p. 3826, 1995, which is based on the idea that information from a deep position can be obtained when R is larger, using light of a light source continuous in time as incident light. For example, using one light source and two detecting elements, information of a shallow part and a deep part is obtained from signals corresponding to each R. Concerning R, depth information of up to 0.35R can be obtained, and the position reaching this depth is the middle point of R. On the other hand, the latter time resolution method requires a light source of narrow time width and a detecting element of fast time response, in order to correspond to depth information by dividing the time response of an optical signal.
However, the conventional biomedical optical device involves, for example, the following problems.
Firstly, signal quality fluctuates depending on a measuring state. That is, in the contact state with the body in the conventional biomedical optical device, intensity or property of the optical signal varies significantly when measuring the light, depending on the contact state of a probe for irradiating light or a probe for detecting the exit light with the skin, or distance or angle. Therefore, depending on whether or not the probe is contacting with the skin, whether or not an air layer exists between the probe and the akin, the signal form may be distorted, and the obtained signal may vary in quality.
Secondly, a measuring range is limited by the method of detection. More specifically, in the case of using the spatial resolution method, a large R is needed when attempted to obtain information of a deep position. For example, information at depth of 5 cm corresponds to R of 15 cm. However, since the human body is made of curves, a position 15 cm apart straightly in distance is away from the skin. On the curve obtained by analysis, a corresponding position on the skin cannot be determined automatically, and there is a limit in analysis of measuring position and depth from the obtained light. The spatial resolution method is effective when acquiring information of two overlaid layers, for example, when distinguishing the skin and the subcutaneous fat, or distinguishing the cranial bones and the cerebral cortex. However, when the thickness of the upper layer is unknown or differs depending on location, trial and error may be needed to set the value of R.
Thirdly, there are limits in the aspects of security and price. That is, in the time resolution method, a light source of narrow time width and a detecting element of fast response are needed, but such a device is expensive, and requires high voltage for driving. For this reason, it is dangerous when brought closer to the skin.
In the conventional devices, therefore, it has been difficult to analyze the measuring position and depth from the measured light, and signals of high quality enough for analysis cannot be acquired, while assuring high safety, if attempted to compose a more inexpensive device than other diagnostic apparatuses.